With development of modern society and people's awareness of environmental protection, more and more equipment use a rechargeable secondary battery as a power source, such as mobile phones, laptops, electrical tools, electrical automobiles and energy storage power stations, which provides a broad space for application and development of the rechargeable secondary battery. Accordingly, safety of power batteries is attracted more and more attention, especially the safety of the power batteries with high capacity which are required by such as electrical automobiles and energy storage power stations.
To ensure the safety of a power battery to a certain extent, generally a cap assembly of a power battery is provided with a vent. When the battery has an accident caused by improper charging, short circuit, or exposure to a poor environment such as a high temperature environment, the battery with high energy will produce a large amount of gas and temperature thereof will rise sharply, the gas bursts through the vent to release pressure, existence of the vent greatly improves safety performance of the battery. But a certain amount of gas will be produced in normal operation or manufacturing process of the battery, and generally a gas pressure of equal to or less than 0.2 MPa will generated. If the vent bursts prematurely under less than 0.2 MPa, there must be too many limitations on production processes and operative conditions of the battery, and what is even worse is that the battery can not be normally produced or can not normally operate. If bursting pressure of the vent is too large, and the vent exhausts and releases pressure too late, battery explosion will be caused.
Some batteries are provided with the vents in cap assemblies in the prior art, which are generally formed by stamping, difference between an upper limit and a lower limit of the bursting pressure of the vent itself is equal to or less than 0.3 MPa based on processing capability of the industry in the prior art. But after the cap assembly is assembled with a case to form the battery, the gas pressure inside the battery increases due to produced gas in the production or operative process of the battery, so as to cause case expansion to produce a force which applies on the cap assembly of the battery, the force is then transmitted to the vent. Namely the vent also bears a drawing force produced by case deformation in a direction parallel to the cap assembly of the battery in addition to an effect by gas pressure in a direction perpendicular to the vent body, if the drawing force is too large, the strength at the vent will be greatly reduced, so as to greatly reduce the bursting pressure of the vent. As the battery is required to have an increased capacity density as much as possible in the prior art, thicknesses of walls of the case and the cap assembly of the battery are more and more thinner, which causes the strength to get worse, so as to increase the probability of occurrence of the above situation that the bursting pressure of the vent is reduced. If an original design requirement on the bursting pressure of the vent is 0.4˜0.7 MPa, because of the above drawing force, the bursting pressure of the vent will be integrally reduced by 0.2˜0.4 MPa, which very likely causes the vent to burst in the production or normal operative process. At the same time, the upper limit of the bursting pressure of the vent is not expected to be increased, because it will greatly increase risk that the vent cannot timely burst and release pressure in an abusing environment, so as to cause firing and explosion of the battery.